1. Field of the Invention
The present invention relates to a zoom lens system, an imaging device, and a camera. In particular, the present invention relates to: a high-performance zoom lens system that has a high resolution and a relatively high zooming ratio and still has a short overall length of lens system (a distance on the optical axis, from an object side surface of a lens element located on the most object side of the lens system, to the image surface) because of a small number of lens elements constituting the lens system; an imaging device employing this zoom lens system; and a thin and compact camera employing this imaging device.
2. Description of the Background Art
Size reduction and performance improvement are strongly required of digital still cameras and digital video cameras (simply referred to as digital cameras, hereinafter) provided with an image sensor for performing photoelectric conversion. Particularly, in recent years, thin digital cameras are required in which top priority is imparted to storability and portability. As one of means for realizing such thin digital cameras, various zoom lens systems have been proposed in which a light beam incident from an object is bent.
Japanese Laid-Open Patent Publication No. 2006-267862 discloses a zoom lens, in order from the object side, comprising: a first lens unit having positive refractive power and including a reflecting member that bends the optical path at about 90 degrees; a second lens unit having negative refractive power; a third lens unit having positive refractive power and including a diaphragm on the most image surface side; and a fourth lens unit having positive refractive power. The second lens unit moves along the optical axis at the time of magnification change, and the fourth lens unit moves at the time of magnification change and focusing. The fourth lens unit comprises a cemented lens having negative refractive power and a positive meniscus lens. The ratio of the focal length of the second lens unit to the focal length of the entire system at a telephoto limit, and the ratio of the focal length of the fourth lens unit to the focal length of the entire system at a telephoto limit, are defined.
Japanese Laid-Open Patent Publication No. 2006-317481 discloses a variable magnification optical system, in order from the object side, comprising: a first lens unit having positive optical power; a second lens unit having negative optical power; a third lens unit having positive optical power; a fourth lens unit having positive optical power; and a fifth lens unit. At the time of magnification change from a wide-angle limit to a telephoto limit, the positions of the first lens unit, the third lens unit, and the fifth lens unit are fixed, and the second lens unit and the fourth lens units move. The fifth lens unit comprises a negative component and a positive component in order from the object side. The ratio of the axial air space between the negative component and the positive component to the axial thickness of the entire fifth lens unit is defined.
Japanese Laid-Open Patent Publication No. 2008-268833 discloses a variable magnification optical system, in order from the object side, comprising: a first lens unit having positive refractive power and being fixed at the time of magnification change and focusing; a second lens unit having negative refractive power and moving at the time of magnification change; a third lens unit having positive refractive power and being fixed at the time of magnification change and focusing; a fourth lens unit having positive refractive power, having a focusing function, and moving at the time of magnification change; and a fifth lens unit having negative refractive power and moving at the time of magnification change. The ratio of the focal length of the first lens unit to the focal length of the entire system at a wide-angle limit, and the ratio of the focal length of the second lens unit to the focal length of the entire system at a wide-angle limit, are defined.
Japanese Patent Publication No. 4264842 discloses a zoom lens including a reflecting member for bending the optical axis passing through a plurality of lens units, and comprising: in order from the object side to the image side, a first lens unit having positive refractive power and being fixed at a position; a second lens unit having negative refractive power and moving along the optical axis at the time of magnification change; a third lens unit having positive refractive power and being fixed at a position; a fourth lens unit having positive refractive power, compensating position variation of the image surface at the time of magnification change, and moving along the optical axis for focusing; and a fifth lens unit having negative refractive power and being fixed at a position at the time of magnification change. The ratio of the focal length of the first lens unit to the focal length of the entire system at a wide-angle limit, and the ratio of the focal length of the third lens unit to the focal length of the entire system at a wide-angle limit, are defined.
However, among the zoom lenses and the variable magnification optical systems disclosed in the above-mentioned patent documents, some have a relatively short overall length of lens system because of a small number of lens elements constituting the lens system but have a low zooming ratio less than 3, while others have a relatively high zooming ratio but have an undesirably long overall length of lens system because of a large number of lens elements constituting the lens system. Thus, these zoom lenses and variable magnification optical systems do not satisfy the requirements for digital cameras in recent years.